A fundamental aim of microscopy and inspection is to generate contrast from an object under examination on a point-by-point basis. Without contrast nothing can be distinguished. In this context, the notion of point-by-point refers to limits of observation as determined by resolution of a system.
The need to provide sufficient contrast to discern details has led to the development of many useful inspection techniques. Examples include dark-field microscopy, phase-contrast imaging, differential interference contrast microscopy, and Schlieren photography.
Within each one of these techniques there are numerous variations, each designed for a particular purpose or situation. The different techniques or variations can provide different information. Thus, multiple techniques or variations are often used to examine an object. This is particularly true when the sample under examination displays characteristics that favor one form of imaging in one region and a different form of imaging in another.
Of the various forms of imaging techniques, the use of scattered light is particularly useful in certain applications. Scattered light gives rise to a signal that is dark-field in nature. If the sample is uniform in composition and topography, then there is no mechanism for the illuminating light to be scattered. On the other hand, a defect or variation can scatter the illuminating light and provide a dark-field signal.
Dark-field imaging can be used to detect small variations in otherwise homogenous surroundings. As an example, dark-field imaging can be used to inspect semiconductor device structures for minute defects that may be in the form of particles on a surface, faults in an otherwise perfect (or almost perfect) array, slight protrusions (or mouse-bites) in line structures, or other defects. These defects are often difficult to discern in a bright-field mode because the signals from surrounding structures are often so strong that they overwhelm the more subtle signals from small defects. Using dark-field imaging in this situation allows the bright background to be largely eliminated so that scattered light from defects can be detected.
There are two main types of dark-field microscopy that are distinguished by their illumination techniques, one uses normal incidence of illumination and the other uses oblique incidence of illumination. In the former, light that is scattered to regions surrounding the illumination aperture is collected. This mode is often referred to as grey-field imaging due to the proximity of the scattered light to the illuminating light.
The oblique incidence technique can be single dark-field or double dark-field. It is referred to as single dark-field if the scattered light is collected in the plane of incidence. It is referred to as double dark-field if the scattered light is collected outside the plane of incidence and to the side, i.e. if the collection space differs from the illumination space in both polar and azimuthal directions (i.e., along different angular directions up and down from the plane of the sample and around a direction perpendicular to the sample).
Conventional oblique incidence dark-field imaging systems use separate components for illumination and collection. For example, the illumination is typically performed by the focusing action of a lens that is held at an angle with respect to the sample, and the collection is typically performed by a separate lens. Such a system requires separate illumination and collection components arranged in fixed positions. These components occupy a certain amount of the available numerical aperture (NA) space above the sample. As such, the NA allocated to illumination is limited. This limits resolution since interrogation spot size (in linear dimensions) is inversely proportional to the NA. Furthermore, using components that are fixed in position restricts collection space to specific regions. Thus, improved systems and methods having increased flexibility are desired.